Interferometry techniques are used for testing aspheric surfaces. In one test configuration, known as a null configuration, a reference wavefront and a test wavefront are formed to produce an interference pattern. Errors in the test measurement are then interpreted directly as errors in the test surface. The Hindle test is an example of a null configuration and may be used for testing convex hyperboloidal secondary mirrors. A difficulty in this approach, however, is that the auxiliary optics are often very large and difficult to fabricate. Moreover, this type of test arrangement is subject to environmental errors.
Recently CGHs have been used to measure optical surfaces or the wavefronts from the optical surfaces. The CGHs usually include patterns of lines which act as diffraction gratings. These patterns are usually written onto, or etched into glass substrates. The CGHs may be written with circular symmetry to preserve the rotational symmetry of most aspheric optics. The circular symmetry type of CGH disperses the diffraction orders along the axis, bringing them to a focus at different axial positions. See “Optical Shop Testing”, second edition, K. Creath and 3. C. Wyant, D. Malacara Ed. (Wiley, N.Y., 1992), pp. 602-612, for more discussion on CGHs.